Abstract [en]

Unlike ab initio calculations, classical potentials in molecular dynamics (MD) allow the treatment of very large atomic systems. However, the quality of the potentials has to be carefully assessed, making the evaluation of the models important. By means of MD simulations, we have studied the properties of liquid iron (Fe) with an embedded-atom method (EAM) under extreme pressure-temperature conditions, to resemble the conditions found in the Earth's outer core. The model used for the MD simulations is demonstrated to reproduce recent experimentally determined structure factor calculations to the highest pressure of 58 GPa. The calculations of densities and diffusion parameters at the various temperatures and pressures agree well with first-principles calculations. The results indicate the model's validity for the physical description of liquid iron at extreme conditions.

Abstract [en]

In this thesis, material properties have been examined under extreme conditions in computer-based calculations.

The research on iron (Fe), nickel (Ni), and ferropericlase (Mg1-xFexO) are not only important for our understanding of the Earth, but also for an improved knowledge of these materials per se.

An embedded-atom model for Fe demonstrated to reproduce properties such as structure factors, densities and diffusion constants, and was employed to evaluate temperature gradients at Earth core conditions. A similar interaction together with a two-temperature method was applied for the analysis of shock-induced melting of Ni. For Mg1-xFexO, the magnetic transition pressure was shown to increase with iron content. Furthermore, the C44 softening with pressure and iron composition supports the experimentally observed phase transition for Mg0.8Fe0.2O at 35 GPa.

The properties of high density helium (He) is of great interest as the gas is one of the most abundant elements in the solar system. Furthermore, He and neon (Ne) are often used as pressure media in diamond anvil cells. The melting of He showed a possible fcc-bcc-liquid transition starting at T=340 K, P=22 GPa with a Buckingham potential, whereas the bcc phase was not seen with the Aziz form. For Ne, Monte Carlo calculations at ambient pressure showed very accurate results when extrapolating the melting temperatures to an infinite cluster limit. At high pressure, a one-phase ab initio melting curve showed a match with one-phase L-J potential results, which could imply a correspondence between ab initio/classical one-phase/two-phase calculations.

In the search for hard materials, ab initio calculations for four TiO2 phases were compared. Just as imposed by experiment, the cotunnite phase was found to be very hard. The anomalous elastic behavior of the superconducting group-V metals V, Nb, Ta was found to be related to shrinking nesting vectors and the electronic topological transition (ETT).